The present invention relates to a slider servo device. particularly to a servo device for a slider which can be moved in the radial direction of an information recording disk (hereinafter also called "the disk") while bearing a pickup for reading recorded information from the disk.
Since the recording track of an information recording disk extends continuously and vertically from the innermost portion of the recording region of the track to the outermost portion thereof, an information reading spot made by a pickup needs to be moved from the innermost portion to the outermost portion. For that reason, a slider servo device for moving the pickup between the innermost and outermost portions of the recording region while maintaining an accurate positional relationship between the information reading spot of the pickup and the recording track of the disk in the radial direction of the disk is indispensable. The slider servo device functions so that a slider which can be moved in the radial direction of the disk while bearing the pickup, is controlled depending upon the low-band component of a tracking error signal corresponding to the quantity of deviation of the reading spot from the recording track in the radial direction of the disk. A wide range of speed is required. The range includes the normal speed of slider movement for normal-speed picture reproduction, the low speed of slider movement for low-speed picture reproduction or still picture reproduction and the high speed of slider movement for triple-speed picture reproduction or searching. In addition the slider needs to accurately and instantaneously follow abrupt changes in the mode (speed) of slider movement.
A slider unit generally has a "dead zone" such that a driving output cannot be generated until the absolute value of the level of the input to the slider unit reaches a prescribed magnitude. For that reason, at the time of the normal-speed picture reproduction or the low-speed picture reproduction or the still picture reproduction, error components are integrated and accumulated in the dead zone. If the servo gain of the slider servo device is too high when a first edge of the dead zone is exceeded, the slider is abruptly moved in the reverse direction (which is such a direction as to make the error zero) so that the second edge, opposite to the first edge, of the dead zone is exceeded. As a result, the slider is again abruptly moved in the new reverse direction, and the first edge of the dead zone is again exceeded. Consequently, the slider goes into a state of oscillation. However, if the loop gain of the slider servo device is set too low, the responsiveness of the slider deteriorates at the time of abrupt application of an error component to the slider in the high-speed movement thereof for the triple-speed picture reproduction, searching or the like. In the worst case, a tracking actuator goes out of the angle of a field of vision so that the slider servo device no longer properly operates.